Science and Nature Writing

Growback 4: Mystery of the dirt rings

Here’s a puzzle for you – one with a prize. Imagine you bought a paddock like this: poor soils, scattered trees, sparse grass, a few eroded patches, bounded by roadside trees.

Now sit back, close your eyes, and imagine that you remove the sheep and cattle and watch the paddock regenerate, without assistance or disturbance. Over 40 years, native trees and shrubs colonize almost the entire area. Birds, kangaroos and other wildlife become more common. From a conservation perspective, a great outcome indeed.

But there’s just one catch. For some reason, big patches of bare soil appear. The bare patches contain no trees or shrubs and few ground plants. The bare soil is crusted and compacted.

Looking at the black and white photo above, can you predict where the patches of bare soil would occur, 40 years in advance? Would they be on the ridges, in the valleys, on the steepest slopes, or would they follow some other pattern?

Over the years, big patches of bare, crusted soil appear.

To find out, let’s leap forward 40 years, and see what happened.

The image below is a composite image of two overlain air photos, one taken in 1971 and one in 2010. The background image is a close-up of the top left corner of the black and white photo at the top of the blog. It was taken in 1971, before regeneration began. The dark spots are the scattered paddock trees. Each tree canopy is about 20 m wide.

The yellow shading is from an air photo taken nearly 40 years after the B&W photo. In 2010, the yellow patches had few ground plants, no trees, no shrubs, just bare, crusted soil. [For the technically minded, the yellow shading comes from a supervised classification of the geo-referenced 2010 image]. All of the areas that aren’t yellow are now covered with native plants.

In 2010, big patches of bare, crusted soil formed the patterns shown in yellow. What caused these patterns to form?

As you can see, the patches of bare soil formed obvious patterns in 2010. Many patches formed wide rings that seem to encircle – but usually were a long way away from – the original paddock trees. These rings don’t seem to match any features on the original air photo, as you can see from the top photo.

This week’s quiz question is – How did these patterns form? What causes the rings? Why aren’t the bare areas covered in plants?

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To give you time to solve the mystery, all will be revealed next week. To encourage lots of guesses, the most accurate and comprehensive comment will win a signed copy of the book, Plains Wandering: Exploring the Grassy Plains of South-east Australia.

The best answer has to explain two things: why do the bare patches occur where they do, and why don’t they occur in other places on the photo?

So put your thinking caps on, and submit your comments below. I’m happy to answer questions to help crack the Mystery of the Dirt Rings.

Acknowledgments: I extend a big thank you to Deanna Duffy at CSU’s Spatial Data Analysis Network, who created the composite image.

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43 thoughts

A question from someone on another continent – is there a particular reason the initial trees were in the locations they were? Are they the remnant uncleared trees from a historically more wooded area or are they responding to soil or moisture conditions of some kind?

Great first question Chris. The initial paddock trees were either retained as shade trees for stock or re-grew after the paddock was first cleared in the 1800s. Before clearing, the area was more densely timbered than it was in the 1970s. I shall have to be a bit cryptic in my replies to this blog, so I don’t give the answer away too soon. Best wishes Ian

I don’t know that I have any answers, but I do notice some things. The first is that the biggest and closest-together trees have the largest “yellow free” areas around them. That to me suggests that the trees are favoring some kind of vegetation.

The second thing that I’m noticing is that the trees tend to have more vegetation in the areas “above” them on my screen. Is the top north? Or south? If so, is the pattern related to the direction of most shade from the slanted rays of the sun?

One pattern I have sometimes observed is that some types of vegetation are associated with tree shade and/or tree roots. Tree roots often go far beyond the drip line (or the shadow) of the tree.

I’m also noticing that the yellow patches tend to be somewhat linear. Could it be that some species of plant (or fungus?) grew just outside the influence of the trees, and did something harsh to the soil, and then died, leaving a temporary empty area.

A last pattern that I notice in the wonderful composite aerial is that the line between vegetation and bare ground is often sharpest closest to the trees. Then vegetated areas resume spottily. I don’t know what that means, but I do find it interesting.

I am not assuming that the bare patches are a bad thing. Perhaps they are a natural feature of this sort of system. Then again, perhaps they’re not. I do wonder if the vegetation close to the trees is similar to the vegetation far from the trees, and I wonder if the vegetation close to the bare areas is similar to the vegetation farther away.

Hi Stephen, great suggestions from a seasoned observer of natural patterns – I especially like your last query. My answers to this blog will be intentionally cryptic, so to your five questions above, I say (1) perhaps, (2) yes, north is at the top, (3) maybe, but not a fungus, and (4) the vegetation close to trees is different to that further away, but your last suggestion isn’t relevant. I hope that helps! 🙂

My guess would be that the patterns are a result of presence/absence of nutrients. The bare crusted soil is where there are low nutirets. The higher nutrient areas are where the cows and sheep congregated under the paddock trees and defecated adding nutrients to the soil, thus supporting more vegetation growth. However, if this were the case, the ground would be more compacted under the trees, and this takes decades to uncompact, which would result in less veg under where the old paddock trees were.

Hi Jono, thanks for a great suggestion. One query though, if this was the case, wouldn’t the bare areas have been visible on the 1970s air photo, before the regeneration began?

You can see my socratic educational theory emerging, as i reply to each suggestion with a question rather than an answer (it drives many students up the wall). I’m happy to pursue your argument further though. Thanks again, Ian

On second thoughts, the pattern doesn’t fit consistently with my first theory. The crusts start too abruptly, right at the edge of probably where the paddock tree roots ended. So perhaps the tree roots have done something to the soil that allows for veg growth….

Hi again Jono, this is a reply to your 2nd comment. You are definitely on the right track here. You’re working towards the right mechanism, but you’ve got the pattern slightly wrong. I think a combination of your comment with suggestions from others will nail the explanation pretty soon, best wishes Ian

How exciting! They remind me of the lichen crusts (or biological soil crusts, I’m not sure of the ‘proper’ name) I’ve seen out in the mallee.
However I don’t know enough about them, or their range to guess correctly – I know they don’t like being trampled, so that would fit with the removal of grazing.

Hi Manuelinor, yes lichens have occupied a lot of the bare soils. There are a few different names for the crusts, including ‘biological soil crusts’ and ‘cryptogamic crusts’. Both terms highlight that lots of different types of organisms grow in soil crusts, including lichens, liverworts, mosses, fungi and algae. These crusts are really valuable as they protect the bare soil surfaces from erosion, which could be a lot worse in this area if the crusts didnt form. Best wishes Ian

I think the pattern of the yellow, bare ish compacted patches result from the historical preferential foraging/eating patterns, and the associated intensity and frequency of the congregating (and compacting) patterns of the livestock, as influenced by the tree canopies.

I’m aware that studies have shown that tree canopies have an influence on soil properties, in particular the enhanced availability of nutrients, which in turn influences herbaceous/pasture productivity and quality. You can get a relatively better quality herbaceous biomass resulting, especially when the overall soil fertility conditions are low. Relying on my memory of some reading I think that the highest concentrations of phosphorus in particular can be found in areas beyond tree canopy projections. If this is so, then the patterns could be due to the relatively better quality of the forage vegetation in these areas; the livestock preferred these areas, spent more time, repeatedly eating in them (no doubt also adding more nutrients from their dung) with their hard hoofs also compacting the soil structure in that process. At some stage the productive quality of the herbaceous pasture must have started to decline with the latter (maybe stocking rates were a factor too?). Subsequent natural regeneration has then been affected – hampered — by this resultant level of compaction. Maybe too the phosphorus and nitrogen levels have played a role in the bare patches. And on another related thought to this… you haven’t mentioned any other management actions other than taking stock off. But if there was an explosion of exotic, weed species at some period some herbicide regime may have also contributed? I also thought of the shade aspect and how it might be combined with the canopy role above, and how that too might affect herbaceous productivity and quality. I tended to discount the role of shade vis a vis compaction by resting cattle directly as the patterns are not also closer under the trees.

Thanks for this great puzzle. It’s been fun to think about on a Sunday morning.

I look forward to finding out the answers – and the range of other explanations.

Hi Deb, thanks, I can see that the photos can stimulate a fantastic range of great hypotheses. A query for you – if livestock compacted the soils and prevented regeneration around the paddock trees, where would you expect the bare soil areas would be – directly beside the trees or further away?

Good thought about herbicides too, but I can rule this one out as it isn’t an issue here. Keep thinking! Thanks again Ian

Hi Ian,
Ecological- prize- winning- quiz format, I hope it catches on! Im going to have a crack with these suggestions. Can I make more than one- the shotgun approach?
1: Nutrient recycling, moisture retention and biological activity may be only occurring in the zones with vegetation to input into the system leaving areas beyond the trees (and regeneration) with few resources. (maybe that’s 3 things)
2: The trees act as shelter protecting the soil from exposure to wind and sun.
3: They would also have provided shelter to sheep and cattle with ?consequences for the soil.
4: The areas around the trees are where seedlings appear and from where regeneration radiates.
5: The type of vegetation in the pasture- maybe exotic grass, weeds? was excluded from the tree zones and that vegetation didn’t persist through the prevailing drought up to 2010. Are other areas with no remnant trees also bare?
6: Your response to Stephen maybe provides a clue that suggests the vegetation is different nearer the bare areas- can I hazard another guess that either certain shrubs – such as Burgan- or regenerating eucalypts may create peripheral bare areas through alleleopathy or high demand for resources….

Hi Antblue, now you know you can’t get a point in a multiple choice test if you put a tick in all the boxes, don’t you? 🙂 Great suggestions though.

Point 1 – this encompasses many of the issues, but I’m looking for a more direct explanation than this – this is a good starting point.

Point 2 – this is true, but it’s not directly relevant to the patterns. Remember that the areas beneath the old trees are under the grey ‘blobs’ (ie the tree canopies) on the photo, not beside them.

Point 3 – true again, but perhaps not relevant to the yellow zones which are a long way from some of the old trees

Point 4 – yep and directly relevant – build on this one too.

Point 5 – I’m not sure that I’ve interpreted your point correctly here. However the bare soil patches are still visible after the drought broke, and were probably visible before the latest drought hit.

Point 6 – this is getting good, and is worthing taking further.

Your suggestions, plus those provided by other readers, provide a good launching pad to a great explanation. Thanks for taking the time to write such a comprehensive comment, best wishes Ian

My guess is salinity. The bare crusted soil patches are patches of saline soil – salt scalds. These are areas where the water table, carrying salt (mobilised from a salt store below ground) has reached the surface: the water has evaporated, leaving the salt on the surface. Hence the bare, crusted ground. The crusting occurs because saline soil has poor structure and basically ‘collapses’ when subject to pressure – such as from rain, forming a crust when it dries. The saline soil does not occur around the established trees because the trees have root systems substantial enough to take up the water before it reaches the surface.

Hello Marie, thanks for another great suggestion. This raises an interesting question… If salinity caused the bare patches, and bare patches formed wherever trees didn’t take up lots of soil water, as you suggest, would you expect all of the surrounding paddocks that haven’t regenerated with native plants to also show lots of serious salinity? As far as I can tell, the biggest bare patches are most obvious in paddocks that have regenerated densely with trees and shrubs, rather than in pastures without regeneration. Any thoughts? Thanks again, I’m really encouraged by the range of great suggestions that have been sent in, best wishes Ian

I’m not really familiar with Australian landscapes…or grasslands, really. From what I see, the immediate areas around the bushes/trees would have the best soil because the roots would stabilize the soils, and also the plants would provide organic matter, making it richer in nutrients. This combined would create more mixing of the soils, which would increase air pockets, and decrease the possibility of soil compaction/salinity problems.
There is usually a ground layer of growth various seedlings of moss or something below/nearby, which also increase the organic layer of the soil. The roots of grassland vegetation are deep and this deep layer creates a lot of soil organic matter (dead root and just better mixing) and its possible that you wouldn’t see this without digging into the soil. So this rich soil will persist while the other soils without large vegetation will not be as developed.
Also I’m assuming that fires burn regularly through this area? The vegetation would again grow in the same places because of the deep roots and the better soil quality. It’s also likely that the plants are fairly fire retardant. Fire can exacerbate soil quality of the areas that don’t have grass/trees, and can destroy seedlings and smaller fauna.

I also agree with salinity and soil compaction for the other places where plants will grow. So even if there is rain, it would just mostly be runoff and not have infiltration.

And another thing: some plants will have those self-thinning biological components. It’s possible that the roots may spread underneath the soil/have seedlings, but they need to be sufficiently far away to flourish or they die, because they’re competing with water/sun. For example, some poplar forests actually grow from the same root, but you can’t see it. So it may be a natural biological way of spreading. You can’t really grow things close together because of competition for water, so it may result in this randomized pattern.

I’m not sure whether the areas with more vegetation would be mounded, (like a forest in Canada), or whether the vegetation started growing in those particular areas because they were sinks where water could gather… I think that was half a question.

Hi FOTW, thanks for a great comment, with lots of good ideas. A couple of clarifications first…. The area hasn’t been burnt for perhaps 100 years now, and maybe not since it was first cleared.

You are right about the effects of trees on soils. The most fertile soils in the area – and in cleared paddocks in Australian in general – are under the big old trees. Falling litter over many years builds up the organic matter, and promotes good water infiltration below old trees, creating ‘fertile islands’ in what is otherwise a pretty infertile area.

Your paragraph about self-thinning and roots is getting close to the mark. Have a look at Jono’s 2nd comment above, as both are starting to angle towards the right answer.

I think if someone was patient enough to compile all of the sections from different comments that are ‘on the right track’ into one big comment, then the answer might soon fall into place. It is a complex pattern to guess, given you haven’t seen all of the evidence from the 2010 photo. Thanks again, best wishes, Ian

Along with the roots answer
It seems that the yellow patches occur probably where root systems from other plants meet, making it nearly impossible for vegetation to grow there (soil nutrients/soil organic matter/ water competition is too high).
It is also possible that the land is slightly sloped and then dips into the area where most of the trees are clumped and the left side of the picture. In 2-fold, it means that the soils will be more stable than the slope, get more water, and it will be easier for roots to form. If there is a slope towards the left (east?), seeds would possibly profligate down slope. The vegetation on the slope that is already there is stabilized, but it might not be possible for larger plants to grow because of the competition for water, mostly, as the trees roots are deeper and larger, while seedlings and understorey need to fight it out to survive. The amount of sunlight can also be a problem on a slope for plants too.
Either that or the yellow areas look a bit ridged (on the aerial photo) but I’m not sure because I think the photo with the yellow patches has slightly different dimensions and may extend a bit more than the aerial photo (or the other way around)
REITERATION:
Where the trees have invested means that there is a soil organic layer from dead leaves/roots), which creates better soils, which have better infiltration. Along with trees, an understorey layer might start growing, where other types of vegetation (moss, forbs, herbs, etc) start growing along with seedlings. This also helps to make the soil richer and helps to stabilize the soils (the roots), and create better air pockets for infiltration however there comes a point where the trees can be too close together and compete for resources, which can result in dry soil, and a place where plants can’t grow (not even lichen).
You also said the area is fairly infertile, which means that the larger woody plants wouldn’t be able to grow too close together (their saplings and seedlings would die) without making too much competition so there could be self thinning, so there would be areas where nothing grows (yellow patches). The trees are relatively small in the aerial photograph but you also said lichen grows there, and there could also be other small plants that grow around the trees that are benefited from the better soils that wouldn’t be in too much competition compared to if trees started growing there

Hi again FOTW, this is a reply to your 2nd comment. I think you’re getting to the issue in the last 2 paragraphs of your comment. I have a feeling that the ecosystem here looks very different to those you know well, which makes your suggestions even more interesting, since you live in a very different biome. I’ll post lots of photos of the site in my next blog, and these might give you a bit of a surprise. I’m delighted that so many readers from different parts of the globe have accepted the challenge – especially since its proving hard enough to answer for the locals! Thanks again for taking the time to write such a detailed comment, best wishes Ian

Thanks to everyone for a great series of suggestions on what is causing the ‘mystery of the dirt rings’. The comments have included many more processes than I had thought of, which is fantastic. Give the breadth of suggestions, it’s perhaps time to provide a couple of CLUES to keep things on track.

The photos were taken from an area in central Victoria that has regenerated abundantly over the last 40 years. The regeneration is dominated by two species, one a tree – the dominant tree is grey box (Eucalyptus microcarpa), and the other a shrub, drooping cassinia (Cassinia arcuata).

Many of the above comments touch on the processes that create the big bare areas. The answer lies in thinking about where these two species might regenerate (on the photo with the yellow patches) and how they might interact.

I hope you all guess it soon! Thanks again to everyone who has taken the time to write a comment so far, best wishes Ian

Thanks for this great puzzle. My guess is: This looks like a source/sink dynamics in a semi-arid ecosystem. If this is an ecosystem with low rainfall, and there’s not enough water to support a continuous vegetation cover, water movement and area’s of infiltration will determine vegetation structure. In a rainfall even the water flows overland, down even the slightest slope, carrying leaf litter and sediments, until it hits a vegetation island with trees, shurbs, grasses and leaf litter. This is where the water infiltrates the soil. These islands are the “sinks,” trapping nutrients, water and seeds. The bare open areas are the “source” of water and nutrients. The bare crusted patches look like biological soil crusts dominated by cyanobacteria and lichens. These sort of biological soil crusts form where water is limiting, soils are stable and/or soils are poor in nutrients. They are common in places that have lost their soil A horizon. Perhaps these paddock lost a lot of soil through overgrazing. The bare crusted patches will not be overgrown be vascular vegetation as the water continues to carry nutrients and seeds away from them and towards the vegetated patches. Also, some lichens in the bare areas may inhibit growth of vascular plants through mechanisms such as allelopathy. So I’d expect the bare rings to occur at a regular distance from the vegetated patches, slightly upslope or on the ridges. You won’t see the bare crusted patches in the “sink” areas because the lichens and cyanobacteria will be out-competed by vascular plants where water is not limiting.

Hi Cassia, great suggestion. I think you’ve captured some of the mechanisms here. I don’t think the upslope/downslope topographic aspect works in this site as strongly as you’ve suggested though (it might work to some extent, but its not very apparent). The ‘solution’ to the puzzle may lie in reversing your sentence, “water movement and areas of infiltration will determine vegetation structure”. How might vegetation structure – and more specifically the two regenerating species – affect water movement and areas of infiltration? The feedback between vegetation and soils is really important in this case. As another clue, you are spot on with your sentence, “I’d expect the bare rings to occur at a regular distance from the vegetated patches”. They key question now is – which vegetation causes the bare patches? Thanks again, it’s fantastic to see the different perspectives that everyone is bringing to the problem. Best wishes Ian

I know that Cassinia arcuata can be invasive in pasture, is favoured by disturbance, and avoided by stock – is poisonous to them? And Euc microcarpa coppices greatly after disturbance including being grazed. If Cassinia got a hold on the areas that are now the dense regen. areas while the paddock was still being grazed by live stock, they would have avoided those areas. Any E. microcarpa that had survived grazing while stock were there would then also grow unimpeded amongst the Cassinia, added to by any new seed and then seedlings blown in from the older established Eucs – all resulting in the dense regeneration described. So these now yellow bare areas could be the areas that ended up being intensively grazed by the stock prior to their removal, to the detriment of the vegetative cover and soil structure and capacity for regeneration. It links back to my original idea of preferential grazing and compaction, but from another angle – but maybe quite off track again.

Hi Deb, thanks for another great suggestion. This makes great sense, but I don’t think its what’s happened in this case. My guess is that the patterns aren’t related to earlier grazing patterns. Instead they are more likely to be related to the regenerating trees and shrubs. How could dense trees and shrubs cause gaps? One point to clarify – in the study area grey box has regenerated abundantly but nearly always from seedlings rather than coppice. I hope all of my replies are helping to clarify things rather than just making it all seem more and more complicated. Its actually pretty simple in the end, but I can only say that because I’ve seen it on the ground, whereas the ‘evidence’ in the puzzle can be interpreted in lots of different ways. Thanks again, best wishes Ian

I notice that there seems to be a drainage line that runs through the paddock site which suggests that at some point, perhaps permanently in the past or intermittently under extreme weather conditions, those lines were filled with water. It’s hard to tell from the photo but I suspect the areas between the distinct drainage lines are also undulating.
My first guess is that the bare patches are areas that are peaks in the landscape that have had the most exposure to wind and rain erosion which has forced the nutrients (and good soil, leaf litter etc.) into the depressions, which is where the trees are.
The lack of uniformity of the rings from the tree edges could therefore be explained by the variation in size or shape of the depression. The old paddock trees would have aided in the re-vegetation from 1971 of the areas immediately surrounding it, leaving the peaks still bare in 2010. The two different tree species & associated vegetation would occupy different levels of depression in the landscape depending on the nutrient/water requirements.
My guess seems simplistic compared the above comments but it was the first thing I noticed.

Hi Jo, yes you’ve read the landscape well; the darker zones nor the 1971 photo are the drainage lines and the paler areas are areas with sparser grasses on the slopes. The area between the drainage lines is undulating as you noticed. I think you’re angling towards the right thing when you said “The old paddock trees would have aided in the re-vegetation from 1971 of the areas immediately surrounding it” but am not sure. However a key feature of the yellow zones is that they don’t follow the topography closely – the reason they have appeared only began after the revegetation started, and it’s not associated with the original topography or the earlier grazing management. So, what processes would start after the revegetation began, which might cause the bare zones to appear?

OK, here’ s my last stab at this. Are we simply? looking at recruitment “halo’s” out from the trees and it is these various halo like configurations of dense native regen. that via root competition result in the bare areas.

Hmm, whadya mean ‘simply’ – its taken a dozen detailed questions to get this far. I think you might have it now, but am not sure. Given we’ve got such a huge prize riding on this, I was hoping for a slightly less ambiguous interpretation than this. Can you say what plants occur on either side of the yellow zones and why no plants, including the two dominant species, occur in the yellow areas? If you can spell it out a little more, you might have it. Looking forward to your next stab! Best wishes Ian

Just venturing a stab guess from Iowa in the U.S. (making this a truly cosmopolitan quiz). Could there be a role for hydrophobic soils associated with the Eucalypts or something related? Yes, we don’t encounter hydrophobic soils in Iowa but I have learned about them from a friend in California. I do know that some Eucalypts shed oils that prevent absorption of water, and that might be a contributor to reduced infiltration and runoff.

Hello Rich, that’s a great suggestion. Hydrophobic soils occur in many areas in Australia, due to the high wax and oil content of eucalypt foliage. It can create a big problem after wildfires as the hydrophobic soils shed water which promotes runoff and erosion. In this case, I don’t know if the bare soils are hydrophobic, although it wouldn’t surprise me if they were. We do know that the soil surface is very crusted and compacted, and that the rate of water infiltration is very slow. But we’d expect this to happen simply because of the long exposure of the bare soils to the sun and rain. Hydrophobicity, if it occurs, would strengthen this effect. If hydrophobic soils are an issue, then we still need to answer the pattern question, which is why do the bare soils occur in obvious zones, and why are these zones spaced where they are? I think the comments are close to answering this now. Your suggestion about hydrophobic soils might be an extra factor that has contributed to causing the patterns to form. Thanks again for taking the time to send in the comment, best wishes Ian

Ok, I’ve looked at the species & soils more closely. Cassinia arcuata is a pioneer species that can grow quickly, occupy shallow infertile soils and compete strongly with native species for light and moisture. Eucalyptus microcarpa prefers loamy moderately fertile soils and will not easily tolerate sandy soils.
So, are the soils within the yellow zone deep and sandy? (which could exclude both species.); soils near the yellow zone relatively infertile but shallow? (excluding E.microcarpa but allowing C.arculata to flourish); soils near the old paddock trees loamy and fertile? (where the E.microcarpa successfully competes against the C.arcuata).

Hi Jo, thanks again. One of the fascinating things about the study site is that, while soils do vary naturally across the site, some of the major soil patterns that now exist are driven by the vegetation itself – thus the vegetation isn’t just responding to natural soil patterns, it’s strongly influencing the soil patterns. All of the vegetation zones in the photos, including areas occupied by the two dominant species and the yellow gap zones, occur across nearly all of the site, and they aren’t associated with any pre-existing soil patterns. The soils are more fertile under the big old trees (which you can see as the black dots on both photos), but this is because big old trees create fertile patches of soils, not because the soils were initially more fertile there. A key to the solution is to think about where the two species are most likely to regenerate, and how they will affect each other, including how they will affect other plants in their vicinity. I hope this doesn’t sound even more confusing 🙂 Thanks again, I look forward to your next great suggestion. Best wishes Ian

Closer to the remnant trees I would think the regen species would be seedlings of those trees, (which you suggest are E. microcarpa) as a seed ‘rain’ of 20 to 30 metres seems reasonable. Beyond this the shrub Cassinia becomes dominant in the regen. with increasing openness/light, and decreasing soil fertility away from the trees although I imagine some tree seedlings could be still present there too. The deeper rooted tree seedlings closer in to the tree canopy spread would allow water movement/infiltration downwards – into the water table; moving outwards from here, and towards the bare patches, the more sparsely foliaged and shallow rooted Cassinia shrubs proliferate, are relatively denser in numbers, and ‘scavenge’ for and utilise any available water (and nutrients). So, there isn’t much depth to infiltration allowed, nor shading to reduce evapotranspiration, or available other resources, and there’s a lot of root competition, making recruitment beyond the zone of their dominance very difficult for other vegetation, and probably also for much if any recruitment of the odd seedling of the Eucs. which might set up a different process across the bare areas. I thought that there could be a role for slope as well but can’t see enough differentiation in the pattern for this.

Wow, getting close, but I’m not sure that everyone will necessarily follow your logic. I wonder how you would answer the following questions (some of which you have above, but others not), in this order. What would regenerate beside the old trees? Is there a way that what ever regenerates besides the old trees could prevent anything from growing in the yellow zone, slightly further out? And then, what would regenerate in the areas beyond the yellow zones, away from the old trees? I think you’ve got the first and last questions right, and have partly answered the 2nd question, but in a round about way. Sorry if it sounds like I’m drawing this out, but the mystery seems too good for me just to blurt out the answer. Thanks again for your persistence, Ian

Regeneration and plant succession radiates out from the paddock trees. As the fertile paddock tree patches increases in size, tree seedling establishment increases until such a point that their density exhausts available resources (most likely soil moisture) and they die off. This occurs at the least fertile edges of the patch in a rough diameter from the main paddock trees. Following this, the species that colonise patches with limited resources more readily, like Cassinia, fill the space. As the regenerated Eucalytus trees pull soil resources in a ring around the paddock trees, the Cassinia must pull soil resources from a ring further outside, ultimately creating bare patches of soil devoid of the resources required by the present species to survive in.
As to the uneveness of the “rings” – it seems as though they are generally all uneven on the one side (north east), so I’d say prevailing winds are from the south west which would increase seedling germination in this direction?

Hi Jo, you and Deb are getting pretty close now. I think you’ve described the vegetation patterns well now, but I’m still slightly unsure how the gap zone is maintained? Can you just clarify how this happens? You probably only need one sentence. Thanks again for following through with getting towards the answer, best wishes Ian

I’m not confident about the maintenance of the gap zones…except to clarify what I suspected in my previous comment.
The resources available to the shrubs & grasses in the areas near gap zones can only be sourced from the gap zones either from soils beneath the crust or precipitation runoff from top of the crusts which may also flush nutrients into this vegetation. The exposed crusts would harden over time facilitating the ongoing void of vegetation growth in these zones.
Perhaps the gaps are also passively maintained by the passage of fauna??

Hi Jo, I think you and Deb have probably gotten as close to the story as is possible based on the clues you’ve seen. Well done! I was planning to ‘reveal all’ in my next blog, with lots of pictures so you can all finally see everything. I have a bit of work to do before this will be finished, so please bear with me for a few days. I’ll also announce the prize winner, which will take some consideration given the great suggestions provided. In the meantime, if anyone would like any part of the story explained further, please say so, and I’ll fill in some of the gaps while you wait for the photo blog.

Thank you to everyone for the amazing array of suggestions you’ve all provided. You’ve broken my (not so huge) record for the most comments I’ve ever had on a blog, and certainly sent in the most detailed comments I’ve ever received.

Next blog – the Mystery of the Dirt Rings will be revealed, in full technicolor. Stay tuned!

Great work Ian, another tantalising question. I completely agree with the resource availability aspects (particularly water) of yours and the above comments. In these landscapes there are sufficient resources available for understorey vegetation to persist op to near the base of the tree, but when these resources are consumed by the mass recruited juvenile trees they suppress the understorey, hence the scaring. In highly resource poor environments the mass recruitment isn’t even required and the ‘initial’ trees/plant/s can produce their own scaring. A great example of this is the State Forest immediately south of Lake Wallawalla, up near Mildura, check out some Google Earth pics. Although I wouldn’t discount allelopathy from impacting these sites as well.

Thanks Chris for your great observations. The bare areas are most obvious at this site where the regen is dense, but the roots zones around single trees suppress grasses too. You can see this on the air photo on the next blog. I’ll have a look at the Mildura site next time I’m on google earth. Thanks again and best wishes Ian

How did these patterns form? What causes the rings? Why aren’t the bare areas covered in plants?
I can only admire the thinking aimed at answers to the first two questions, which leaves me guessing as to why the bare areas are not covered with plants.
Assuming soils are infertile, a contributing factor might be topography. The original B+W aerial photo shows an uneven topography, where the crests appear to coincide with the gaps and the rings on the later aerial composition. The resulting denudation could be caused by prolonged pressure from a combination of surface runoff and subsoil leaching, making this site a prime candidate for massive sheet and rill erosion.